Her comments come after the Metropolitan police said a man in his 50s had been held on suspicion of indecent assault, sexual assault, rape, abused of position of trust and misconduct in public office.
The unnamed Tory MP is accused of offences spanning seven years and has now been released on bail pending further inquiries.
The Metropolitan Police said officers received a report in January 2020 relating to alleged sexual offences committed in London between 2002 and 2009.
Ms Truss said she was “very, very concerned” that an MP had been arrested on suspicion of rape and said it is “a matter for the police”.
Speaking to Sky News, she said: “I’m very, very concerned about the reports that I’ve heard. Clearly this is a matter for the police.
“The police are currently working on the case but it is worrying that we are seeing these appalling … accusations, again, about a parliamentarian.”
Asked if the MP should have been suspended or had the whip withdrawn, she said: “I think it’s a matter for the police about whether that individual is named.
“I don’t know who it is. I know that the police are looking at it and investigating it and I think we need to wait for the police to conduct those investigations.”
Tory chief whip Chris Heaton-Harris urged the MP to stay away from Parliament but has not suspended the whip, meaning the suspect remains a member of the parliamentary party.
The Met Police said officers received a report in January 2020 relating to alleged sexual offences committed in London between 2002 and 2009, and the investigation is still ongoing.
It was understood the Conservatives will not make a decision on whether the suspect has the whip suspended until after the police investigation is concluded.
‘Firmer action is needed’
The news comes amid renewed scrutiny of sleaze in Westminster following a string of cases.
Tory MP Imran Ahmad Khan resigned as an MP last month after he was found guilty of sexually assualting a 15-year-old boy.
David Warburton had the Conservative whip withdrawn after allegations of sexual harassment and cocaine use emerged.
Neil Parish also stood down as the Conservative MP for Tiverton and Honiton earlier this month after admitting watching pornography in the Commons.
Over the past year, around 15 MPs are suspected to have been investigated by Parliament’s Independent Complaints and Grievance Scheme (ICGS) watchdog that examines allegations of bullying, harassment or sexual misconduct.
The Prospect union, which represents hundreds of staff working in the Houses of Parliament, said firmer action is needed.
Deputy general secretary Garry Graham said: “What will it take for Parliament to finally take its responsibility to its staff and visitors seriously and suspend access to the estate for parliamentarians under investigation for sexual offences?
“Voluntary agreements to stay away do not work, as demonstrated by Imran Ahmad Khan’s attendance at Westminster whilst investigations were ongoing, despite agreeing to stay away.
“Parliament has the same responsibilities towards its staff as any other workplace and it must live up to them.”